Method for Recovering Methane Gas from Natural Gas Hydrate

ABSTRACT

The present invention relates to a method for recovering methane gas by adding a gas mixture containing N 2  and CO 2  gases to natural gas hydrate and reacting them. The method for recovering methane gas from natural gas hydrate of the present invention comprises the step of replacing CH 4  gas in natural gas hydrate with a gas mixture containing N 2  and CO 2  gases by adding the gas mixture to the natural gas hydrate. The method for recovering methane gas of the invention assures a recovery rate of CH 4  gas much higher than prior art method without dissociating natural gas hydrate layer and utilization of flue gas as a gas mixture containing N 2  and CO 2  gases, which makes possible its practical application for the production of natural gas in terms of economy and environmental protection.

RELATED APPLICATION INFORMATION

This application claims priority under 35 U.S.C. §119(b) to KoreanApplication Serial No. 10-2006-0071835, filed Jul. 31, 2006.

FIELD OF THE INVENTION

The present invention relates to a method for recovering methane gasfrom natural gas hydrate, more specifically, to a method for recoveringCH₄ gas by adding a gas mixture containing N₂ and CO₂ gases to naturalgas hydrate and reacting them.

BACKGROUND OF THE INVENTION

Natural gas is eco-friendly energy source playing an important role inthe energy system which has been maintained without environmentaldisruption. Natural gas exists as natural gas hydrate in nature, whichis a crystalline clathrate hydrate containing hydrocarbons such as amajor component of methane, and trace amounts of ethane, propane andbutane. Natural gas hydrate deposited worldwide is being noted as analternative energy source to fossil fuel in the future, since naturalgas in a form of natural gas hydrate has been reported to be stored inan amount of about 0.2×10¹⁵ to 7,600×10¹⁵ m³, which is an amount tosupply most energy consumed in the world (see: Sloan, Jr. E. D., et al.,Nature, 426:353-359, 1998; Lee, S. Y., Holder, G. D., Fuel ProcessingTechnology, 71:181-186, 2001).

To maximize the utilization of such valuable natural gas hydrate as anenergy source in real life, methods for dissociating CH₄ from thenatural gas hydrate should be explored, and several approaches have beenmade in the art as follows: thermal stimulation which dissociatesmethane from the natural gas hydrate by injecting water of hightemperature to hydrate layer through a pipe; depressurization whichdissociates methane from the natural gas hydrate by depressurizingnatural gas hydrate layer using a vacuum device; and, injection ofinhibitors which dissociate methane from natural gas hydrate byinterfering the condition that natural gas hydrate exists in a stableform to change the equilibrium toward higher temperature and lowerpressure (see: Gunn, D. A., et al., Terra Nova, 14:443-450, 2002).

However, in case of using the said prior methods, CH₄ gas as a majorcomponent of natural gas hydrate is dissociated and released directly sothat natural gas hydrate layers are disrupted, to give rise togeological problems such as a ground subsidence and environmentalproblems such as considerable changes in the ecosystem (see: Lelieveld,J., et al., Nature, 355:339-342, 1992).

SUMMARY OF THE INVENTION

The present inventors have made an effort to develop a method forrecovering CH₄ gas from natural gas hydrate without dissociating thenatural gas hydrate layer, and discovered that CH₄ gas can be recoveredefficiently by adding a gas mixture containing N₂ and CO₂ gases tonatural gas hydrate under a high pressure to replace CH₄ gas in naturalgas hydrate with the gas mixture by the difference in the partialpressures of CH₄ gas in the natural gas hydrate and the gas mixture.

A primary object of the present invention is, therefore, to provide amethod for recovering methane gas by adding a gas mixture containing N₂and CO₂ gases to natural gas hydrate to replace CH₄ gas in natural gashydrate with the gas mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and the other objects and features of the present inventionwill become apparent from the following descriptions given inconjunction with the accompanying drawings, in which:

FIG. 1 is a diagram depicting the replacement of CH₄ gas in natural gashydrate with a gas mixture containing N₂ and CO₂ gases by adding the gasmixtures to the natural gas hydrate.

FIG. 2 is a graph showing the time course of the recovery rates of CH₄gas, in case of adding a gas mixture containing N₂ and CO₂gases(experimental group) and CO₂ gas only(control) to natural gashydrate.

DETAILED DESCRIPTION OF THE INVENTION

Methane(CH₄) gas forms methane gas hydrate having a cubic structure(Pm3n, 8CH₄.46H₂O) when reacting with water at 0° C. under a pressure ofabout 30 atm, which occupies most of the natural gas hydrate layers(see:Kvenvolden, K. A., Chem. Geol., 71:41, 1998). The present inventors havemade various efforts to develop a method for recovering methane gas fromnatural gas hydrate efficiently without dissociating the natural gashydrate layer, while focusing on the availability of CO₂ gas.

Methane gas hydrate is formed by reacting CH₄ gas with water at 0° C.under a pressure of over 25 atm, whereas CO₂ gas hydrate is formed byreacting CO₂gas with water at 0° C. under a pressure of over 12 atm. Inthis connection, it has been reported that carbon dioxide hydrate existsstably under a condition that methane gas hydrate exists in a stableform, while methane gas hydrate cannot exist stably under a conditionthat carbon dioxide hydrate exists in a stable form(see: Sloan, Jr. E.D., Clathrate Hydrates of Natural Gases, 2^(nd) ed., Marcel Dekker, NewYork, 1998). The present inventors carried out a series of experimentsunder an assumption that most of CH₄ gas can be recovered by adding CO₂gas to natural gas hydrate under a condition that forms carbon dioxidehydrate, to replace CH₄ gas in the natural gas hydrate with a gasmixture without dissociating the natural gas hydrate layer, by thedifference in the partial pressures of CH₄ gas in the natural gashydrate and the gas mixture, and succeeded in recovering CH₄ gas with arecovery rate of 64% (see: Lee Heun, et al., Angew. Chem. Int. Ed.,42:5048-5051, 2003).

However, the recovery rate was considered not reach to the theoreticallyexpected one, therefore, the present inventors further performedexperiments to elucidate the cause of the said difference, and foundthat the said cause was uneven distribution of CH₄ gas in the naturalgas hydrate in small cage or large cage. That is, though there is nodifference in molecular size of added CO₂ gas, CH₄ gas whose molecularsize is smaller than CO₂ gas, can be readily replaced with CO₂ gas inthe large cage of the natural gas hydrate, while it cannot be made inthe small cage, and such a phenomenon gives a recovery rate lower thanthe theoretically expected one.

Under the circumstances, the inventors tried to increase the recoveryrate while overcoming the above-mentioned problem, made an assumptionthat the addition of a gas mixture containing N₂ and CO₂ gases, whichcan be entrapped in a small cage in a form of hydrate, to a natural gashydrate may give the recovery rate of CH₄ gas higher than that of CO₂gas only (see: FIG. 1). FIG. 1 is a diagram depicting the replacement ofCH₄ gas in natural gas hydrate with a gas mixture containing N₂ and CO₂gases by adding the gas mixture to the natural gas hydrate. Further,they made an assumption that: N₂ gas having a particle size smaller thanCO₂ gas can replace CH₄ gas even in a cage having a size that CO₂ gascannot replace, however nitrogen hydrate, due to its unstable nature,cannot be fixed in a natural gas hydrate without CO₂ gas; and,therefore, N₂ gas would be preferably used in a mixed form with CO₂ gas.

A series of experiments to replace CH₄ gas in natural gas hydrate with agas mixture containing N₂ and CO₂ gases were carried out to convince theassumptions made above, and addition of a gas mixture containing N₂ andCO₂ gases to the natural gas hydrate, gave a recovery rate of CH₄ gasmuch higher than that of using CO₂ gas only. Particularly, it wasdemonstrated that: usage of a gas mixture containing N₂ and CO₂ gases ina molar ratio of 8:2, at the point of 9 hrs reaction, gave a recoveryrate of CH₄ gas (64%) which is maximal in case of using CO₂ gas only; asthe time passed over 9 hrs, it gave a recovery rate of CH₄ gas higherthan that of control; at the time of 20 hrs reaction, it gave a maximumrecovery rate(85%); and, at the point of 20 hrs reaction, the recoveryrate did not increase any more. Such results were the same as the caseof using gas mixtures containing N₂ and CO₂ gases in a molar ratio of9:1 to 1:9.

Considering economical and environmental aspects of the method ofpresent invention, a gas mixture containing N₂ and CO₂ gases may besubstituted with flue gas exhausted from factories. Flue gas whichcontains a variety of gas components such as CO₂, N₂ gases, water andsulfur, is regarded as an example of the gas mixture containing N₂ andCO₂ gases, since the gas components other than N₂ and CO₂ gases arereadily removed by using a cleanup apparatus as a means for protectingenvironmental pollution.

Under a consideration that the composition of flue gas is similar tothat of the gas mixtures stated above and the gas mixture containing N₂and CO₂ gases in a molar ratio of 1:9 to 9:1 can be employed in themethod of present invention, the flue gas was employed as an alternativeto the gas mixture in the replacement reaction, which gave a recoveryrate of CH₄ gas ranging 71% to 83%. Usage of the flue gas, which is anindustrial waste exhausted from the factories, makes possible itspractical application for the production of natural gas in terms ofeconomy and environmental protection, though it has revealed ashortcoming of unsteady recovery rate of CH₄ gas due to the considerablevariation in the composition ratio of N₂ and CO₂ gases. Further, itprovides a benefit of countering global warming, since CO₂ gas containedin the flue gas can be removed in the atmosphere.

As stated above, a method for recovering methane gas from natural gashydrate of the present invention comprises the step of replacing CH₄ gasin natural gas hydrate with a gas mixture containing N₂ and CO₂ gases byadding the gas mixture to the natural gas hydrate. In carrying out themethod, the gas mixture, but not limited thereto, contains N₂ and CO₂gases in a molar ratio of 1:9 to 9:1, more preferably 4:6 to 9:1, mostpreferably 8:2, and flue gas may be used in stead of the gas mixture.Further, the gas mixture, but not limited thereto, is added to thenatural gas hydrate at a temperature of 0 to 5° C. to reach a pressureof 30 to 200 atm and reacted for 9 to 20 hours. The temperature of 0 to5° C. is a range that the natural gas hydrate exists in environmentalcondition, and recovery rate of CH₄ gas from the natural gas hydrate isdecreased under a temperature of below 0° C., and replacement of CH₄ gaswith gas mixture is not made normally under a temperature of above 5° C.since the natural gas hydrate become unstable. In addition, replacementof CH₄ gas with a gas mixture can be made efficiently under a pressureof 30 to 200 atm without dissociating natural gas hydrate, and recoveryof CH₄ gas from the natural gas hydrate cannot be made since thestructure of natural gas hydrate is dissociated under a pressure ofbelow 30 atm.

The present invention is further illustrated by the following examples,which should not be taken to limit the scope of the invention.

EXAMPLE 1 Recovery of CH₄ Gas from Natural Gas Hydrate by Addition of aGas mixture Containing N₂ and CO₂ Gases

Natural gas hydrate was subjected to a high-pressure cell for Ramananalysis in accordance with the method described in publication of theinventors(see: Sloan, Jr. E. D., Clathrate Hydrates of Natural Gases,2^(nd) ed., Marcel Dekker, New York, 1998). While maintaining theinitial temperature of 0° C., a gas mixture containing N₂ and CO₂ gasesin a molar ratio of 8:2 with the same temperature was added to thehigh-pressure cell to reach a pressure of 120 atm in the cell to replaceCH₄ gas in the natural gas hydrate with the gas mixture for 23 hrs. Thetemperature of the high-pressure cell at the point of 23 hrs was 1.1° C.And then, the amounts of CH₄ gas recovered from the natural gas hydratewere measured as time goes by, and the recovery rate of CH₄ gas wascalculated by comparing them with the amount in CH₄ gas contained in thenatural gas hydrate, where the amount of CH₄ gas was measured by the aidof Raman spectrometer(RFS-100S FT-Raman Spectrometer, Bruker, USA) andCO₂ gas was used as a control(see: FIG. 2). FIG. 2 is a graph showingthe time course of the recovery rate of CH₄ gas in a case that CH₄ gasis recovered from natural gas hydrate by using CO₂ gas only(control) anda gas mixture containing N₂ and CO₂ gases(experimental group), where (•)and (▪) indicate control and experimental groups, respectively.

As can be seen in FIG. 2, the recovery rates of CH₄ gas in case of usinga gas mixture containing N₂ and CO₂ gases were changed as follows: atthe point of 9 hrs reaction, the recovery rate was the same as that ofcontrol; as the time passed over 9 hrs, the recovery rates were higherthan those of control; and, at the point of over 20 hrs reaction, therecovery rates were not increased any more. Further, in case of using agas mixture, the recovery rate of 85% was measured finally, which wasmuch higher than that (64%) of using CO₂ gas only (control).

From the above results, it was considered that more CH₄ gas, in case ofusing a gas mixture, could be recovered from natural gas hydrate, sinceCH₄ gas entrapped in the small cage was replaced with N₂ gas, unlike thecase of adding CO₂ gas only to recover CH₄ gas.

EXAMPLE 2 Determination of Ratio of N₂ and CO₂ gases Contained in Gasmixture

Based on the results of Example 1 that more CH₄ gas could be recoveredfrom natural gas hydrate by using a gas mixture containing N₂ and CO₂gases, the present inventors examined whether the recovery rate waschanged depending on the ratio of N₂ and CO₂ gases contained in the gasmixture: Recovery rate of CH₄ gas was measured similarly as in Example1, with the exceptions of using a gas mixture containing N₂ and CO₂gases in a molar ratio of 0:10, 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2or 9:1 and reacting for 20 hrs (see: Table 1).

TABLE 1 Changes in recovery rate depending on the ratio of N₂ and CO₂gases in a gas mixture Mixing Ratio of N₂ and Recovery Rate of CO₂ (inmolar ratio) CH₄ gas(%)  0:10 64 1:9 71.2 2:8 74.4 3:7 79.6 4:6 82.9 5:581.1 6:4 78.4 7:3 84.2 8:2 85 9:1 83.3

As can be seen in Table 1 above, it was found that: as the N₂ gascontent in a gas mixture increased, the recovery rate of CH₄ gas wasincreased and the highest recovery rate was achieved in case of using agas mixture containing N₂ and CO₂ gases in a molar ratio of 8:2; and,the recovery rate of CH₄ gas was much higher than the case of using CO₂gas only, even though the gas mixture contained a small amount of N₂gas.

Accordingly, it was clearly demonstrated that CH₄ gas could be recoveredby using a gas which contains N₂ and CO₂ gases in a variety of mixingratios more efficiently than by using CO₂ gas only.

EXAMPLE 3 Recovery of CH₄ Gas by Employing Flue Gas

Based on the results of Examples 1 and 2 that gas mixtures containing N₂and CO₂ gases could be used for the recovery of CH₄ gas, the inventorsexamined whether flue gas containing N₂ and CO₂ gases in a variety ofmixing ratios could be used for the recovery of CH₄ gas from natural gashydrate: Recovery rate of CH₄ gas was measured similarly as in Example1, with an exception of using flue gases collected from 10 factorieslocated at an industrial complex instead of the gas mixture (see: Table2).

TABLE 2 Changes in recovery rate of CH₄ gas in case of using a varietyof flue gases containing N₂ and CO₂ Flue Gases Mixing Ratio of N₂ andRecovery Rate of from CO₂ (in molar ratio) CH₄ gas(%) Factory 1 6:4 78.1Factory 2 7:3 83.4 Factory 3 4:6 81.5 Factory 4 6:4 80.4 Factory 5 2:874.1 Factory 6 3:7 78.5 Factory 7 5:5 81.3 Factory 8 1:9 71.1 Factory 95:5 80.6 Factory 10 1:9 71.3

As can be seen in Table 2 above, it was clearly demonstrated that arecovery rate of CH₄ gas in the range of 71% to 83% could be obtained byreplacing CH₄ gas with a variety of flue gases containing N₂ and CO₂.

EXAMPLE 4 Determination of Pressure Condition in a Gas Mixture

To determine the pressure condition in a gas mixture, recovery rate ofCH₄ gas was measured similarly as in Example 1, with the exceptions ofusing a gas mixture containing N₂ and CO₂ gases in a molar ratio of 8:2and reacting for 20 hrs, and subjecting the gas mixture to a pressure of10 atm, 20 atm, 30 atm, 40 atm, 50 atm, 80 atm, 120 atm, 160 atm or 200atm, respectively (see: Table 3).

TABLE 3 Changes in recovery rate of CH₄ gas depending on the pressure ingas mixtures Pressure of Gas Recovery Rate of Mixture(atm) CH₄ gas(%) 10— 20 — 30 83 40 84 50 85 80 84 120 85 160 85 200 85

As can be seen in Table 3 above, it was found that CH₄ gas in a certainamount could be recovered from natural gas hydrate by subjecting a gasmixture to a pressure of above 30 atm, however replacement of CH₄ gaswith the gas mixture in natural gas hydrate could not be made since thestructure of natural gas hydrate was dissociated under a pressure of 10to 20 atm.

Accordingly, it was clearly demonstrated that replacement of CH₄ gaswith a gas mixture could be made efficiently by subjecting the gasmixture in natural gas hydrate to a pressure ranging from 30 to 200 atm.

As clearly described and demonstrated above, the present inventionprovides a method for recovering methane gas by adding a gas mixturecontaining N₂ and CO₂ gases to natural gas hydrate and reacting them.The method for recovering methane gas of the invention assures arecovery rate of CH₄ gas much higher than prior art method withoutdissociating natural gas hydrate layer and utilization of flue gas as analternative to the gas mixture containing N₂ and CO₂ gases, which makespossible its practical application for the production of natural gas interms of economy and environmental protection.

1. A method for recovering methane gas from natural gas hydrate whichcomprises the step of replacing CH₄ gas in the natural gas hydrate witha gas mixture containing N₂ and CO₂ gases by adding the gas mixture tothe natural gas hydrate.
 2. The method for recovering methane gas fromnatural gas hydrate of claim 1, wherein the gas mixture contains N₂ andCO₂ gases in a molar ratio of 1:9 to 9:1.
 3. The method for recoveringmethane gas from natural gas hydrate of claim 1, wherein the gas mixturecontains N₂ and CO₂ gases in a molar ratio of 4:6 to 9:1.
 4. The methodfor recovering methane gas from natural gas hydrate of claim 1, whereinthe gas mixture contains N₂ and CO₂ gases in a molar ratio of 8:2. 5.The method for recovering methane gas from natural gas hydrate of claim1, wherein the gas mixture is added to the natural gas hydrate at atemperature of 0 to 5° C. to reach a pressure of 30 to 200 atm andreacted for 9 to 20 hours.
 6. A method for recovering methane gas fromnatural gas hydrate which comprises the step of replacing CH₄ gas withthe gas mixture for 9 to 20 hours by adding a flue gas containing N₂ andCO₂ gases to the natural gas hydrate at a temperature of 0 to 5° C. toreach a pressure of 30 to 200 atm.